This invention relates to the generation of ionized air for removal of charges resident on surfaces of semiconductor chips during their manufacture and, more particularly, to the use of a sharply focused intense laser beam to ionize air in the vicinity of a semiconductor chip for discharging a surface of the chip.
During the construction of semiconductor chips having electrical circuitry thereon, the construction process entails the deposition and etching of materials as a succession of layers are built up on a surface of the chip substrate. The succession of layers must be kept free of contaminants to ensure proper operation of the chip circuitry. A primary source of contamination is the presence of particulates carried by air in the presence of the semiconductor chips. The particulates are found even in clean rooms employed for the manufacture of semiconductor chips. Typically, the clean room is provided with high efficiency filters (known as HEPA filters) for removal of airborne particles from incoming air. However, particulates are still found in clean rooms. Particulates generated in clean rooms are charged, generally. Also, charges develop upon semiconductor surfaces during the manufacture of the chips. The charges on the chip surfaces produce electric fields which create a strong attraction between the airborne particles and the corresponding oppositely charged semiconductor surfaces. This phenomenon is the primary cause for the anomalously large deposition rates of particulates of small particle size found in chip manufacture.
A technique that has been employed to reduce the foregoing attraction of particulates is to neutralize surfaces of the chip and of the various layers present in the manufacturing process, as well as to neutralize surfaces of tools employed in the manufacturing assembly line, by adding air ions to the output air stream from the foregoing filters. The technique of neutralization employs the passage of ionized air over the semiconductor surfaces. For example, typical electric fields produced by ungrounded wafers or containers are a few hundred to a few thousand volts per centimeter. The deposition rate for particles of class 100 air (100 particles per cubic foot of air) is approximately 100 times lower for environments that incorporate air ionization than for environments that do not employ the ionization. This is attributed to the neutralization effect of charges in the ionized air upon excess surface charges on the semiconductor chips. The discussion of the particle deposition rate in ionized air is presented in an article entitled "Equivalence Between Surface Contamination Rates And Class 100 Conditions" by R. Welker, 1988 Proceedings of the Institute of Environmental Sciences, Pages 449-454.
The presence of charges on the semiconductor surface presents an apparent danger to the minute circuit elements present on the chip. A charged region on the surface of a wafer attracts small dust particles out of the air which adhere to the surface and create imperfections within the chip structure during development of subsequent layers of material in the construction of the chip circuit. Due to the doping of semiconductor materials, such a silicon substrate used in the manufacture of a wafer, by way of example, from the point of view of static electricity, the surface of the wafer has sufficient conductivity to be regarded as an equipotential surface. Contact of the semiconductor surface with an electrically conductive medium may result in a sudden discharge current flowing at a point on the semiconductor surface. Frequently, the magnitude of such current is sufficient to damage or burn out elements of the chip circuit. The voltage, relative to a ground, resulting from triboelectricity may be very large; for example, the voltage resulting from ordinary handling of the wafer during manufacture may be as high as thirty kilovolts. It is also noted that the dust particles in the air are so small as to be carried about in the air by Brownian motion; gravity has little effect on such small particles and has little effect in inducing a settling of these particles on surfaces of the chip or tools used in fabricating the chip. Settling of the particles is accomplished by way of electrostatic forces which attract the particles to surfaces of the chip and the tools. Some of the dust particles arise from materials which may be employed in the chip manufacture.
By way of example, a plastic wafer boat holding a set of wafers might be impregnated with carbon particles to introduce electrical conductivity to the boat for grounding the wafers and reducing surface charges. However, the presence of the carbon would be a source of carbon dust. Thus, the introduction of ionized molecules in the air, such as an ionized oxygen molecule, is most useful in removal of the surface charges without acting as a source of further contaminating dust particles. The ionization process produces both positive and negative ions so as to be capable of neutralizing both negatively and positively charged regions on a surface of the chip. The ionization of the air may be viewed as introducing an electrical conductivity which bleeds off charge from the wafer, as by grounding.
Use of the corona tips for ionizing air introduces particulates, such as sputtered metal, and ions which are carried by the air to impinge upon chip surfaces. Ammonium nitrate may precipitate on the points and, thereafter, separate from the points as dust particles. The generation of ionized air by corona tips located distant from the wafer surface loses its effectiveness in confined spaces, as in a wafer stepper employed for photolithography, because the physical shapes of elements of the stepper introduce turbulence to air flow within the stepper. The turbulence encourages combination of positive and negative ions and, therefore, precludes the transport of balanced ionized air to the wafer surface from a distant source of ions.
A problem with the introduction of ionized air, wherein the ionization is produced by use of corona discharge to produce the ionization, is that the ionized air itself introduces considerable particulate contamination due to the corrosion of electrode tips used in the corona discharges. This is disclosed in an article entitled "Effectiveness Of Air Ionization In Clean Rooms" by M. Suzuki, I. Matshuhasi, and I. Izumoto, 1988 Proceedings of the Institute of Environmental Sciences, Pages 405-412. Generally, the corona discharge results in a negligible amount of particles generated in typical clean-room systems in the 0.1 to 1.0 micron range; a substantial amount of particles of smaller size is produced. The production rates of the smaller particles having diameters in the range of 0.03 to 0.1 microns are between 100 and 1000 particles per cubic foot per minute. The dominant generation mechanism appears to be a sputtering of the corona tips as disclosed by R. P. Donovan, P. A. Lawless and D. D. Smith in an article entitled "Polarity Dependence of Electrode Erosion Under DC Corona Discharge", Microcontamination, May 1986, pages 38-49. In the process of ionization by use of the corona tips, the corona tips serve to concentrate locally the electric fields to a sufficient intensity for exceeding the ionization threshold of the air.
However, in view of the introduction of particulate matter by the corona tips, there is a loss in effectiveness of the resulting ionized air as a means for discharging the semiconductor and the tool surfaces in the prevention of contamination of the semiconductor chips.